This invention relates to image-receiving sheets which are suitable for performing recording by means of dot-shaped heating printing means such as thermal heads, used in combination with a heat transfer sheet having a colorant layer containing a sublimable dye formed thereon, and are excellent in light resistance after recording.
A heat printing system is known in which a dye receivable resin such as thermoplastic polyester resin is laminated as the receiving layer on a substrate sheet, such as paper, and the heat transfer sheet is superposed on the thus prepared image-receiving sheet for effecting heat printing thereby to express gradation of the color depending on the magnitude of imparted heat energy during printing. Further, various hues can be reproduced by a combination of various colors with the use of a plurality of heat transfer papers with different hues during printing, whereby the printed matter in which the same tone as in natural color photography or color printing is expressed can be obtained. The thermal energy for printing can be controlled by electrical signals based on the images recorded by VTR, etc. or projected onto a color cathode-ray tube, and therefore is useful as the system for the so-called "hard copying system" in which these images are taken out as the printed matter. In this case, the thermal heads of the printer are used as the heat printing means, and multiple color dots of 3 colors or 4 colors are transferred by heating within a very short time, whereby the full color image of the original is reproduced with the color dots of said multiple colors.
The color image thus formed is very sharp, because the colorant employed is a dye, and also exhibits excellent transparency, and therefore the image obtained is excellent in reproducibility and tone of the intermediate color, which is similar to the image according to off-set printing or gravure printing, and an image of high quality comparable to full color photographic image can be formed.
However, since the image obtained in the image-receiving sheet of the prior art is formed of a dye, it is generally inferior in light resistance as compared with the image by use of a pigment, and there is involved the problem that the image will be rapidly faded or discolored when exposed directly to sunlight.
For overcoming such drawbacks, there has been proposed an image-receiving sheet comprising a UV-ray absorber or a photostabilizer contained in the receiving layer. As the UV-ray contained in this kind of image-receiving layer, use of, for example, salicylic acid derivatives, benzophenone compounds, benzotriazole compounds and acrylate compounds have been known, while as the photostabilizer, naphthylamine compounds, diphenylamine compounds and phenol compounds have been known.
However, although a considerable effect can be obtained by the addition of these UV-ray absorbers, etc. as compared with the case where no such additive is added, the effect is not yet satisfactory.
Also, the problem of discoloration and fading occurs in other cases than by direct sunlight irradiation. For example, discoloration and fading by indoor light, or discoloration and fading under the state where no direct light such as of the contents of alubum case or book is irradiated may be generated, and these problems of indoor discoloration and fading or dark fading cannot be solved by use of UV-ray absorbers or antioxidants in general, both remaining as the important tasks to be solved.